As is well recognized in the art, thin glass substrates which have been chemically-strengthened by an ion-exchange process are widely utilized in electronic devices, primarily as cover glasses on the displays of smart phones and tablets. Ion-exchange is a chemical process where host alkali metal atoms within the glass of a smaller ionic radius, typically sodium or lithium, are substituted at the atomic level by invasive alkali metal atoms of a larger ionic radius, typically potassium. Ion-exchange is conventionally conducted by immersing glass substrates in a salt bath, or tank of molten salt, containing potassium nitrate (KNO3). The host alkali metal ions vacate from the glass surface region and the larger invasive alkali metal ions wedge into the voids causing the volume of the glass surface region to expand. Provided the temperature is below that at which the glass network structure can relax, a shallow but high-level of compressive stress is formed in the glass surface region. This compressive stress increases surface hardness to resist the formation of scratches, and forces closed microscopic flaws at or near the surface thereby reducing the likelihood of crack propagation on impact or load and thus greatly enhancing glass strength.
Glass substrates for chemical-strengthening by ion-exchange may be of any one of a number of alkali containing recipes where smaller host alkali metal ions are available in the glass surface region for substitution. Traditional soda-lime silicate glass, that which is encountered in common window glass, may be chemically-strengthened by ion-exchange. Other alkali containing glass recipes including alkali-aluminosilicate glass, alkali-borosilicate glass, alkali-aluminoborosilicate glass, alkali-boron glass, alkali-germinate glass, and alkali-borogermanate glass may also be chemically-strengthened by ion-exchange. The alkali-aluminosilicate glass may be a sodium alkali-aluminosilicate, or the less common lithium alkali-aluminosilicate, specifically formulated for “high ion-exchange” with sodium or lithium host-alkali metal atoms readily available in the surface region for rapid substitution. Such alkali-aluminosilicate glass recipes more quickly achieve high levels of surface region compressive stress (CS) and high depths of compressive layer (DOL) during the ion-exchange process.
Thin alkali containing glass substrates are currently manufactured by one of two primary methods or variants thereof, the fusion process and the float process.
The fusion process pioneered by CORNING® Incorporated of Corning N.Y. is used to produce thin substrates of alkali-containing glass, namely of sodium alkali-aluminosilicate recipes, which are commercially available in a thickness ranging from 0.4 mm to 2.0 mm. These substrates are collectively known by the trademark name of GORILLA® Glass after being subject to chemical-strengthening by ion-exchange. The fusion process is an overflow down draw method where molten glass flows around a forming structure, or isopipe, creating two downwardly moving ribbons of glass which are fused into a single glass ribbon at the bottom of the forming structure, or root of the isopipe. The fused glass ribbon is pulled vertically downward away from the isopipe by a system of guide rollers while cooling. Upon cooling at the bottom end of the draw, individual glass substrates are cut from the vertically moving fused glass ribbon by a travelling anvil method to become raw sheets suitable for dimensional fabrication and strengthening by ion-exchange.
The fusion process manufactures thin glass substrates of good flatness and excellent optical quality. The opposing top surface regions of the molten glass ribbons which proceed downward on both sides of the isopipe and become the major outer surface regions of the fused glass ribbon are processed free of contact in the molten state and remain ultimately pristine. However, the fusion process is a slow and expensive process which is difficult to control across larger widths, for example greater than 2,000 mm, or when producing longer substrates which increase the weight of glass suspended beneath the isopipe. Cutting the downwardly moving glass ribbon requires steps be taken to minimize forces traveling upstream to where the ribbon remains in a softened state. The glass ribbon especially if wide or thin may be deliberately curved during the fusion process to simplify drawing but at a penalty of imparting differential annealing histories to the opposing glass surface regions. During later ion-exchange this differential results in a mild asymmetry of salt-ion diffusion between the opposing surface regions. One surface region is mildly “treatment-advantaged” compared to the other surface region being mildly “treatment-disadvantaged”, both in the quantity of salt-ions entering the glass surface region and the depth to which such salt-ions progress.
The float process is also used to produce thin substrates of alkali containing glass. The Pilkington subsidiary of Nippon Sheet Glass Co., Ltd. (NSG) of Japan produces thin substrates of a soda-lime silicate glass recipe in thicknesses less than 3.0 mm to as thin as 1.0 mm thickness. These substrates in a thickness of 1.6 mm and thinner are collectively known by their trademark names of MICROFLOAT™ and MICROWHITE™ depending on the amount of iron present in their composition. Additionally, the Asahi Glass Co., Ltd. (AGC) of Japan has pioneered the use of the float process to produce thin substrates of a ‘high ion-exchange’ sodium alkali-aluminosilicate recipe which are commercially available in a thickness ranging from approximately 0.1 mm to 2.0 mm. These substrates are collectively known by their trademark names of DRAGONTRAIL® and LEOFLEX® after being subject to chemical-strengthening by ion-exchange. The float process is a horizontal production method where molten glass flows over a weir and onto the top of liquid tin metal, or a float bath, from where it is pulled as a ribbon which may be further thinned by additional drawing. The horizontally moving glass travels through an annealing lehr (i.e., a temperature-controlled kiln for annealing glass objects) and is then cut into raw sheets suitable for dimensional fabrication and strengthening by ion-exchange.
The float process allows the manufacture of thin glass substrates of excellent flatness and good optical quality. The glass ribbon can be larger widths, for example 3,300 mm, and since the cutting process occurs many meters downstream from where the softened ribbon of glass is exiting the float bath, substrates may be readily cut in longer lengths without impact to upstream glass. Furthermore, the float process allows for the efficient production of high glass tonnages at low cost. However, substrates produced by the float process suffer from a distinct and ubiquitous problem, a microscopic layer of tin remains embedded in the glass. While tin from the bath can be found in both major surface regions of float produced glass, the lower surface region in direct contact with the metallic tin bath, the tin side, acquires substantially more tin contamination than the upper surface region, known in the art as the non-tin side. During later ion-exchange this differential results in a substantive asymmetry of salt-ion diffusion between the opposing surface regions. As a result, the non-tin side surface region is “treatment-advantaged” compared to the tin side surface region being “treatment-disadvantaged”, both in the quantity of salt-ions entering the glass surface region and the depth to which such salt-ions progress.
Larger invasive salt-ions crowd into the surface regions of the glass substrate during ion-exchange compressing the surface regions and causing a simultaneous expansion in their volume. When the salt-ion uptake is asymmetrical between the opposing major surface regions then the expansion of each major surface region occurs by differing amounts. Both expanded surface regions pivot about a central region of tension with the resulting dimensional differences being accommodated by deformation of the thin glass substrate into a curved body (also referred to as bow or bend or warpage). That is, the asymmetry of salt-ion diffusion during ion-exchange causes thin chemically-strengthened glass substrates to develop a curvature, deviating in shape from that of a true flat plane. Curvature may be defined as the difference in distance on the z-axis exceeding that of glass thickness between higher and lower points on the substrate from an imaginary flat plane bisecting the thickness centerline. The differential tin contamination of the surface regions in thin float produced glass causes a curvature which is an order of magnitude greater than that which occurs due to differential annealing histories on the surface regions of fusion drawn glass. Indeed, typically when a thin substrate of sufficient size is made by the float process, following ion-exchange, it becomes noticeably concave in shape on the tin-side, convex in shape on the non-tin side, and thereby resembles a shallow dish.
Outside obvious aesthetic requirements for flatness, control of curvature out-of-plane in thin chemically-strengthened substrates is a definitive functional requirement for many glass applications. For touch displays, a thin glass substrate is generally assembled as a component to a multi-layer stack where curvature may cause gapping between layers resulting in irregularities of luminance or Newton rings. For electronics or solar applications, curvature may complicate the adhesion and quality level of applied films or coatings such as Indium Tin Oxide. Architectural and transportation applications typically require thin chemically-strengthened glass substrates be laminated to another substrate of glass, or adhered to an object, for which curvature may cause edge curl or ripple formations. Even when thin glass is used as a layer within an insulating glass unit (IGU) or vacuum insulating glass (VIG) to create an additional hermetically sealed void, a warped substrate may experience a washboard effect where the direction of curvature reverses under load or the sidewalls of cavities are in unacceptable contact.